1. Field of Invention
The invention relates to a transmission mode detector for digital receivers and, more particularly, to a transmission mode detector compliant with the ETS 300 401 standard.
2. Related Art
According to the ETS 300 401 standard, a transmission frame is consisted of a null symbol and several successive OFDM (Orthogonal Frequency Division Multiplex) symbols. A receiving system has to detect the presence of the null symbol first in order to verify the presence of broadcasting radio signals. The timing of the detected null symbol is also used as a synchronization signal. Furthermore, there are four transmission modes specified by ETS 300 401 for different broadcasting networks. For example, the transmission mode I is usually used in terrestrial broadcasting in urban areas, while signals broadcasted through satellites are typically in the transmission mode II or IV. TABLE 1 lists the system parameters of the four transmission modes.
TABLE 1  Parameter  Mode I  Mode II Mode III  Mode IVL767615376K1536384192768TF96ms24ms24ms48msTNULL˜ 1.297ms˜ 324μs˜ 168μs˜ 648μsTS˜1.246ms˜ 312μs˜ 156μs˜623μsTu1ms250μs125μs500μsTgi˜246μs˜62μs˜ 31μs˜ 123μs
The system parameters, such as the carrier number K in each OFDM symbol, the symbol number L in each transmission frame, the symbol duration Ts, and so on, are different in different modes. Since the receiving system cannot correctly decode the received data without the information of the transmission mode, the transmission mode must be first detected before the receiving system performs demodulation and decoding.